The invention relates to a process for attaching carbon blocks, which are employed in the fused salt electrolytic process for producing aluminum, to the iron spades or stud means, of an anode suspension means making use of an adhesive substance which is mechanically strong and is a good electrical conductor at least at temperatures between 900.degree. and 1000.degree. C.
For many years now the attachment of anodes in the aluminum industry has been by casting-in the anode studs or spades with cast-iron or by ramming in a special graphite-containing mass.
In the age of the side-feeding non-hooded reduction cells the method of ramming was often considered the best, economical solution in spite of its disadvantages. Repeated break-down in quality in the ramming mass, its sensitivity to operating temperatures, and the high standard required with respect to tolerances at the anode studs or spades often led to bonding problems and a loss in electrical conductivity at the place of attachment.
In the meantime some important factors have changed. Material and energy prices have risen drastically and more severe environmental laws on fume emission mean that hooded, center or point fed cells are almost the only ones that can be employed today.
As a result the more secure, highly electrically conductive means of attaching anodes has become much more important in recent times, both for economic and for operational reasons.
The alternative casting-in method offers some advantages in this respect, but also presents some weaknesses which have to be overcome:
The desired composition of cast iron used for this purpose must be observed and the melt supervised at all times.
If not properly removed, splashes of metal or overflow metal can reduce metal quality.
Induction furnaces, wiping devices etc. incur higher investment and energy costs.
Described in the European published patent application EP-A No. 27 534 is a carbonaceous contact paste for creating a strong, highly electrically conductive connection between elements at elevated temperatures. The paste which is prepared cold comprises a mixture of epoxy resin and tar as binder, and graphite and metal powder as solid additives. Before use an acid-free hardener, e.g. a polyamine, is added to the mixture. The electrical and thermal conductivity of this contact paste corresponds to the conductivity of carbon blocks and is also suitable for example for bonding anode blocks to the suspension means.
The object of the present invention is to improve the viscosity and the mold filling capacity of the adhesive substance for bonding carbon anode blocks to the iron studs or spades of the anode suspension means, and to do so without causing premature hardening. As such, especially in the case of old, heavily worn studs or spades there should be almost no rejects due to deficiency in bonding and, at the same time, the contact resistance between anode stud or spade and anode block should be less than 50 .mu..OMEGA..